1. Field of the Invention
The present invention relates to a control method for an injection molding machine, configured to produce molded products by feeding a resin into a heating cylinder, melting and measuring the fed resin in the heating cylinder, and injecting the measured molten resin into a mold mounted in a mold clamping device, and more particularly, to an advance control method for a screw or plunger for injecting the molten resin in the heating cylinder into the mold.
2. Description of the Related Art
In an injection process, a molten resin injected from a nozzle into a mold flows through a resin channel, including a sprue and runner, and finally reaches production sections, that is, cavities in which molded products are directly shaped. Gates are arranged between the resin channel and the production sections to separate them. Thus, the molten resin injected from the nozzle flows into the cavities, i.e., production sections 16, through a sprue 10, runner 12, and gates 14, as shown in FIG. 1. The sprue and runner portions will be collectively referred to as the “runner”.
The resin channel in the mold comprises various parts with different shapes. Further, the molten resin is a viscoelastic body. Even if the resin is injected at a constant speed, therefore, the pressure on the resin does not constantly increase as the resin is filled into the mold, but sharply or slowly increases depending on the shapes of regions where the resin passes. Further, this pressure change causes the speed of passage of the resin through these regions, possibly resulting in molding failures.
The molding failures include flow marks, that is, traces of molten resin flows that appear as stripes around the gates, and so-called jetting such that the molten resin injected into the cavities through the gates is solidified in the shape of strings and produces meandering patterns on the surface of each molded product. These molding failures are mainly attributed to too high an inlet velocity of the molten resin that flows into the cavities or the production sections through the gates.
A method of preventing jetting is disclosed in Japanese Patent Application Laid-Open No. 7-299850. According to this method, a plunger is stopped from advancing for a set period of about 0.1 second at its set advanced position at the point in time when a runner is fully loaded or a gate portion is reached by the leading flow end of a molten resin, that is, the leading end of a flow of the molten resin injected and flowing into a mold. Thus, generation of peak pressure in the runner is suppressed so that jetting can be prevented.
According to the method described in Japanese Patent Application Laid-Open No. 7-299850, moreover, the position of a screw and the pressure in an injection cylinder are liable to variation for each molding cycle, so that it is difficult to reliably produce high-precision molded products. Thereupon, Japanese Patent Application Laid-Open No. 2008-74114 discloses a control method in which the screw is held short of a position where cavities are filled up with the resin and a packing process is started after the cavities are filled up.
In the method described in Japanese Patent Application Laid-Open No. 7-299850, the pressure on the molten resin in the runner is reduced by stopping the plunger for the set period at the position where the gate portion is reached by the leading flow end of the resin as the runner is filled up with the resin. Means for extruding the molten resin from a heating cylinder during an injection operation, including the plunger, is referred to as the screw. FIG. 2 is an explanatory diagram illustrating the relationship between the screw position during the injection operation and the leading flow end position of the molten resin of the prior art described above. In FIG. 2, the abscissa represents time, and the ordinate represents the position of the leading flow end of the molten resin along a resin distribution channel in the runner. Symbol P denotes the screw position; RP, the position of the leading flow end of the molten resin along the resin distribution channel in the runner; P1, a screw position at the start of injection; and RP1, a position of the leading flow end at an injection starting position. Symbol P2 denotes a screw advance stop position; T1, a screw advance stop time; T2, a screw advance restart time; and ST, a screw advance stop period. Symbol RP2 denotes a position where the leading flow end position RP of the molten resin that fills the runner reaches the gate portion. Symbol A represents a section where the molten resin is flowing into a runner portion, and symbol B represents a section where the molten resin is flowing into the cavities or the production sections.
At the start of injection, the screw starts to advance at the injection starting position P1. The starting point of a screw stroke is set on the tip side of the heating cylinder. On the other hand, the resin in the runner starts to move with a delay after the action of the screw, due to its elasticity and the like. Accordingly, the leading flow end RP of the molten resin starts to move with a slight delay and flows into the runner that serves as the resin distribution channel, as indicated by the section A. The screw advance is stopped at the time T1 for the position RP2 where the gate portion is reached by the leading end of the resin as the runner is filled up with the resin. Symbol P2 represents the screw position at this time T1. Thus, when the started screw advance is stopped at the time T1, the screw position is at P2, and the runner is filled up with the molten resin so that the gate portion position RP2 is reached by the leading flow end position RP. When the screw advance is stopped at the time T1, the runner is filled up with the compressed resin in an excessive volume.
When the screw advance is stopped, the pressure on the molten resin in the runner, that is, the pressure from the screw applied through the resin in the heating cylinder, is reduced, and the resin in the runner is released from compressive force and subjected to a lower pressure. Just after the screw advance is stopped, however, the pressure on the molten resin in the runner is high. Since a space in the runner communicates with the cavities or the production sections, the molten resin released from the compressive force increases its volume as it approaches the cavities through the gate portion. This expanded molten resin passing through the gates flows rapidly into the cavities. In FIG. 2, the section B represents a state in which the compressed molten resin is expanded and flows into the cavities or the production sections. Thus, the molten resin flows rapidly into the cavities while the screw advance is stopped. Thereafter, the screw advance is resumed for an injection operation at the time T2 for the passage of the set screw advance stop period ST.
The inlet velocity of the molten resin that flows into the cavities or the production sections through the gate portion can be kept lower according to the prior art method described in Japanese Patent Application Laid-Open No. 7-299850 than in the case where the resin is continuously injected from the runner portion with the screw advance stopped at a set position for a set period. Nevertheless, this conventional method cannot achieve the object to prevent the resin from flowing rapidly into the cavities or the production sections.